The invention relates to a display device as defined in the precharacterizing part of claim 1.
The invention also relates to a cathode ray tube which is suitable for use in a display device.
Such a display device is used in, inter alia, television displays, computer monitors and projection TVs.
A display device of the kind mentioned in the opening paragraph is known from U.S. Pat. No. 5,270,611. U.S. Pat. No. 5,270,611 describes a display device comprising a cathode ray tube which is provided with a cathode, an electron beam guidance cavity and a first electrode which is connectable to a first power supply means for applying the electric field with a first field strength E1 between the cathode and an exit aperture. The electron beam guidance cavity comprises walls in which, for example, a part of the wall near the exit aperture comprises an insulating material having a secondary emission coefficient xcex41. Furthermore, the secondary emission coefficient xcex41 and the first field strength E1 have values which allow electron transport through the electron beam guidance cavity. The electron transport within the cavity is possible when a sufficiently strong electric field is applied in a longitudinal direction of the electron beam guidance cavity. The value of this field depends on the type of material and on the geometry and sizes of the walls of the cavity. In a steady state, the electron transport takes place via a secondary emission process so that, for each electron impinging on the cavity wall, one electron is emitted on average. The circumstances can be chosen to be such that as many electrons enter the entrance aperture of the electron beam guidance cavity as will leave the exit aperture. When the exit aperture is much smaller then the entrance aperture, an electron compressor is formed which concentrates a luminosity of the electron source with a factor of, for example, 100 to 1000. An electron source with a high current density can thus be made. An accelerating grid accelerates electrons leaving the cavity towards the main electron lens. A main electron lens images the exit aperture of the cavity on the display screen and, via a deflection unit, a raster image is formed on the display screen of the tube.
In a conventional television system it is desirable that the characteristics of the three electron beams for R,G, B are known for performing color point stabilization, black current stabilization and white level stabilization. Therefore, the electron beam current has to be measured at regular intervals at a predetermined drive level during generation of a measurement line in a blanking period. This blanking period is at the beginning of each field. Normally, the image is displayed on the cathode ray tube with some overscan, so that the borders of the image fall outside the visible area of the display screen. However, when an image with a 16:9 aspect ratio is displayed on a display screen with a 4:3 aspect ratio, the measurement line becomes visible. This results in annoying effects on the display screen or the application of adaptations of the vertical deflection to avoid these effects. These annoying effects will also appear in computer monitors, in which the image is displayed with underscan on the cathode ray tube.
It is, inter alia, an object of the invention to provide a cathode ray tube in which the beam current can be measured without visible effects on the display screen. This object is achieved by the cathode ray tube according to the invention, which is defined in claim 1. When the display device in accordance with the invention is in operation, in the blanking period, the switching means are arranged in such a way that the current from the cathode remains uninterrupted, whereas the electron beam is deflected and cannot reach the exit aperture of the electron beam guidance cavity. Therefore, for example, the modulating voltage versus beam current characteristics of the cathode ray tube can be measured during the blanking period without visible artefacts, whereas the beam current is uninterrupted in the display period.
A further advantage is that, with the measured beam current, further operations might be possible such as beam current limitation in order to protect overload of a high tension power supply or geometrical compensation of the image for varying loads of the extremely high tension power supply. Further advantageous embodiments are defined in the dependent claims.
A particular embodiment of the display device according to the invention is defined in claim 2. In this embodiment, the electron beam is deflected between the third electrode and the exit aperture of the electron beam guidance cavity in dependence upon an applied voltage difference between the first and the third electrode.
A further embodiment of the display device according to the invention is defined in claim 3. The addition of the fourth electrode allows a quick start-up of the electron transport mechanism of the electron beam in the electron beam guidance cavity to the display screen with respect to the embodiment comprising only a third electrode, because no negative charge is accumulated on the insulating wall near the exit aperture in the embodiment with the third and fourth electrode when the beam current is prevented from passing through the exit aperture. In this embodiment, a transport voltage on the first electrode is maintained at a constant level.
A further embodiment of the display device according to the invention is defined in claim 5. With the first range of the modulating voltages, a diode characteristic of the cathode ray tube is obtained for a predetermined set of dimensions and shapes of the second electrode and the third electrode, the distance between the cathode and the second electrode, and the distance between the second electrode and the third electrode, respectively. An advantage of this embodiment is that the modulating voltage at the cathode may be in the range between 0 and 10 V so that low voltage electronics can be applied. However, the gamma of the cathode current versus modulating voltage is limited to about 1.8 in this embodiment.
A further embodiment of the display device according to the invention is defined in claim 7. For this second range of the modulating voltages, a triode characteristic of the cathode ray tube is obtained for a predetermined set of dimensions and shapes of the second electrode and the third electrode, the distance between the cathode and the second electrode, and the distance between the second electrode and the third electrode, respectively. An advantage of the triode characteristic is that the gamma of the cathode current versus modulating voltage resembles that of a conventional cathode ray tube so that the cathode ray tube with the electron guidance cavity is more compatible with the conventional cathode ray tube. The gamma is, for example, about 2.4.
A further embodiment of the display device according to the invention is defined in claim 9. A funnel-shaped exit aperture allows hop entrance of electrons with a small electric force in the tangential direction with respect to the exit aperture. In this embodiment, the average energy of the electrons is hardly increased and the spread of energy distribution will also hardly increase, while the spot size on the display screen can be reduced.